Photoynamic therapy (PDT), matured as a feasible medical technology in the 1980s at several institutions throughout the world, is a third-level treatment for cancer involving three key components: a photosensitizer, light, and tissue oxygen. It is also being investigated for treatment of psoriasis and acne, and is an approved treatment for wet macular degeneration. The German physician Friedrich Meyer–Betz performed the first study with what was first called photoradiation therapy (PRT) with porphyrins in humans in 1913. Meyer–Betz tested the effects of haematoporphyrin-PRT on his own skin.[citations needed]